The present invention relates to a process for producing fine metal oxide particles and, more particularly, to a process for producing fine metal oxide particles by heat-treating an aqueous solution of a metal salt under the subcritical to supercritical conditions for water.
The fine metal oxide particles are used in various fields, for example, as a material of ceramics, electronic devices, catalysts, cosmetics and coatings, reinforcing materials and fillers for modifiers, and various methods of producing the fine metal oxide particles are conventionally known.
The fine metal oxide particles used in the above fields are required to have a narrow particle size distribution width and the uniform shape, to be free from a dendrite or a crystal twinning, and to have few secondary agglomerates. The fine metal oxide particles are sometimes required to be uniformly agglomerated or amorphous to a great extent.
Such fine metal oxide particles are conventionally produced by coprecipitation method, heat hydrolysis method or autoclaving method. In the coprecipitation method, the particle size is greatly influenced by the temperature, concentration, charging ratio, charging rate, method of supplying the raw materials, and stirring, so that the particle diameter is not always uniform, resulting in a wide particle size distribution width. To prevent this, various means are taken such as the addition of various additives and the aging of the fine particles after the completion of the reaction.
In the heat hydrolysis method and the autoclaving method, an autoclave is used and a comparatively long reaction time or aging time such as 30 minutes to 48 hours is necessary.
That is, no process for producing the fine metal oxide particles with high productive efficiency has not been established.
As a result of various investigations by the present inventors, it has been found that by heat-treating an aqueous solution of a metal salt under the subcritical to supercritical conditions for water fine metal oxide particles having excellent characteristics such as narrow particle size distribution width and uniform particle shape can be obtained in a short time. The present invention has been achieved on the basis of this finding.